When signal light is transmitted in an optical communication system at a high speed of not less than 40 Gb/s, an optical pulse width of a transmitted signal is narrowed to be several picoseconds. Accordingly, waveform distortion caused by minimal chromatic dispersion or polarization-mode dispersion of an optical fiber significantly deteriorates transmission properties. Further, it has been known that a dispersion value of a transmission fiber is time-varied with a change in the temperature and environment and the minimal change affects the transmission properties.
FIG. 1 is a diagram illustrating an optical receiver in a WDM transmission system in the related art using a technique of chromatic dispersion compensation or a polarization mode dispersion compensation. As illustrated in FIG. 1, an optical receiver 100 includes an optical preamplifier 101 that collectively amplifies WDM light and a demultiplexer 102 that demultiplexes the light into light of each wavelength. Signal light CH1, CH2, . . . , or CHn of each wavelength output from the demultiplexer 102 is supplied to a corresponding optical receiver module 103_1, 103_2, . . . , or 103—n, where reception processing is performed. A function component 111 such as a tunable dispersion compensator (TDC), a polarization-mode dispersion compensator (PMDC), or the like is provided on the optical path in each of the optical receiver modules 103_1 to 103—n to execute a preferable dispersion compensation with respect to the signal light received by each function component 111.
In the case where the function component 111 such as TDC, PMDC, or the like is used, when the power level of the received light becomes low owing to optical loss in the function component 111, a bit error rate (BER) increases in a demodulator 112 and an identification reproducer 113. In order to suppress the increase in BER, it is necessary for each of the optical receiver modules 103_1 to 103—n corresponding to the respective wavelengths to have a function for compensating the optical loss in the function component 111 by providing an optical amplifier 114 between the function component 111 and the demodulator 112. However, when using the optical amplifier 114, there is a possibility that the waveform of the received light is deteriorated owing to noise light such as amplified spontaneous emission (ASE) generated when the optical amplifier 114 amplifies the signal light.
Further, waveforms of the received light may be deteriorated not only by the influence of noise light in the optical amplifier 114, but also by the increase in the width of each optical spectrum when transmitting each signal light at a high speed. Specifically, for example, as illustrated in FIG. 2, when a bit rate of each signal light included in WDM light is increased from 10 Gb/s to 40 Gb/s, the spectrum width of each signal light is increased about four times. At that time, when a distance between channels of the WDM light is narrowly set (50 GHz in the example illustrated in the drawing), there is a possibility that bottom areas of optical spectrums of adjacent channels are overlapped to each other and a cross talk may occur between the channels.
That is, when the optical spectrum width is increased with the increase in the speed of modulating each signal light included in the WDM light, unnecessary light (overlapped components of the optical spectrums) that deteriorates reception properties of each signal is relatively increased.
Herein, since it is desirable to increase the transmission properties without respect to the noise light generated in the optical amplifier 114, a difference between a WDM optical amplifier that collectively amplifies multiple wavelengths, and a single wavelength optical amplifier that amplifies a single wavelength will be described. The WDM transmission system includes a large number of optical amplifiers for WDM (for example, the optical preamplifier 101 in FIG. 1, and the like) that collectively amplify multiple wavelengths on the optical path through which the WDM light propagates, in addition to the optical amplifier for a single wavelength arranged preceding the receiver corresponding to each wavelength. As illustrated in FIG. 3, noise light (see upper part of FIG. 3) such as ASE generated in the optical amplifier for WDM passes the demultiplexer 102 of the optical receiver 100, so that only the noise component exists in the band of each signal light is transmitted through the demultiplexer 102 and supplied to the demodulator 112 corresponding to each signal wavelength, and the noise component exists outside the band is blocked by the demultiplexer 102 (see middle part of FIG. 3). Consequently, the noise light generated in the optical amplifier for WDM does not affect much the reception properties of the signal light of each wavelength, and may be ignored in many cases.
On the contrary, since the optical amplifier 114 for a single wavelength is provided on the optical path through which signal light of each wavelength demultiplexed by the demultiplexer 102 propagates, noise light generated over a wide wavelength band in the optical amplifier 114 is directly input to the demodulator 112 (see lower part of FIG. 3). Accordingly, the ratio of power of the signal light of a single wavelength to the total power of the noise light becomes small, causing the reception properties to deteriorate.
Prior arts related to the present technique include technologies disclosed in the following patent documents. Japanese Laid-open Patent Publication No. 08-321805 discusses a technique that an optical transmission system has a characteristic adjustment device for adjusting a characteristic value of the optical signal. Japanese Laid-open Patent Publication No. 2004-179799 discusses a technique that an optical receiver has an optical filtering device for restricting the band of each channel signal light.